Process gases were sampled from the outlet of a tungsten chemical vapor deposition (CVD) reactor into an Inficon Composer™ acoustic sensor for in situ chemical gas sensing and real-time film thickness metrology. Processes were carried out on an Ulvac W CVD cluster tool at 10 Torr from 340 to 400 °C using a H2/WF6 gas mixture. Sampled gases were compressed through a diaphragm pump up to 100 Torr as required for accurate measurements in the acoustic cell. The high depletion of the heavy WF6 precursor (up to 30%) generated a significant variation of the average gas molecular weight and consequently of the mass-dependent resonant frequency measured by the acoustic sensor. The monitored signal was integrated over the process time, and the integrated area was correlated to the deposited W film thickness determined by ex situ measurements. The average error on this in-tool and real-time metrology was less than 1% over 30 wafers processed, either under fixed process conditions or while varying key process variables such as deposition time or temperature. A dynamic physically based simulator was also developed to validate the system response under different process conditions and demonstrate the fundamental understanding of this method. The metrology achieved represents a significant improvement over previously published data [L. Henn-Lecordier etal, J. Vac. Sci. Technol. A 19, 621 (2001)] obtained on the same system but in the sub-Torr process pressure regime, where low depletion rates (around 3%) had limited the metrology to 7% error. With an error less than 1%, this in situ chemical sensing approach could be efficiently exploited for real-time course correction, e.g., using end-point film thickness control. © 2003 American Vacuum Society.